Metal–Organic Framework (MOF) Derivatives as Promising Chemiresistive Gas Sensing Materials: A Review

The emission of harmful gases has seriously exceeded relative standards with the rapid development of modern industry, which has shown various negative impacts on human health and the natural environment. Recently, metal–organic frameworks (MOFs)-based materials have been widely used as chemiresistive gas sensing materials for the sensitive detection and monitoring of harmful gases such as NOx, H2S, and many volatile organic compounds (VOCs). In particular, the derivatives of MOFs, which are usually semiconducting metal oxides and oxide–carbon composites, hold great potential to prompt the surface reactions with analytes and thus output amplified resistance changing signals of the chemiresistors, due to their high specific surface areas, versatile structural tunability, diversified surface architectures, as well as their superior selectivity. In this review, we introduce the recent progress in applying sophisticated MOFs-derived materials for chemiresistive gas sensors, with specific emphasis placed on the synthesis and structural regulation of the MOF derivatives, and the promoted surface reaction mechanisms between MOF derivatives and gas analytes. Furthermore, the practical application of MOF derivatives for chemiresistive sensing of NO2, H2S, and typical VOCs (e.g., acetone and ethanol) has been discussed in detail.


Introduction
With the recent rapid development of cities and industry, volatile organic compounds (VOCs) and other harmful gases are being released at an increasing rate, which upon entry into the air destroys the environmental and ecological balance [1,2]. For example, NO x causes photochemical smog, SO 2 causes acid rain and haze, and HCHO causes inflammation and cancer, each of which is harmful and has a noticeable impact on human health [3][4][5][6]. Sensors, which are monitoring devices that convert monitored information into electrical signals or other information, have many advantages, such as simple operation, facile pretreatment, low cost and real-time detection compared with traditional gas analysis instruments such as gas chromatographs and mass spectrometers [7][8][9][10][11][12]. In previous studies, electrochemical sensors, optical fiber sensors, and capacitive sensors have been applied for the detection of harmful gases [13][14][15][16][17]. However, these sensors have a series of disadvantages, such as low sensitivity, complex design, and high prices. Chemiresistive gas sensors, as a kind of promising chemical gas sensor, are ohmic contact resistors with two electrodes displaying resistance changes when sensing materials come into contact with the analyte, and the nature of the chemical reaction and the concentration change of the analyte can be inferred by this resistance change [18,19]. Chemiresistive gas sensors avoid the shortcomings of other types of gas sensors commonly used in previous research due to their several advantages, such as having a fast response speed, high sensitivity, easy-to-design Int. J. Environ. Res. Public Health 2023, 20, 4388 2 of 14 structure, simple operation, and low price; thus, they have been widely used in recent years. It is important to choose gas sensors that are suitable for the recording and monitoring of chemical stimuli and variations in the environment in order to study the changeable trends of harmful gases and in turn set more effective pollution control plans, control the emissions of gas pollutants, and reduce their negative effects. The microstructure and surface state of sensitive materials can influence the performance of gas sensors. Extraordinary structures can increase the number of surface active sites and accelerate the reaction rates [20][21][22][23]. For example, a large contact area caused by hollow or porous structures can increase the degree and rate of chemical reactions. Lee et al. studied the acetone sensing properties of ZnO nanoparticles with different oxygen vacancies [24]. The study showed that the existence of oxygen vacancies brought about an excellent response because controlling the surface defects represented a way to change the sensing properties. O 2 in the air adsorbs on the surface of ZnO nanoparticles and combines with the electrons above to form O 2 − , wherein the oxygen vacancy can be used as an electron donor [25][26][27][28][29][30]. When the sensor is exposed to acetone, O 2 − reacts with acetone, and the adsorbed electrons are then released back into the conduction band (CB), resulting in the reduction of the depletion layer thickness and resistance. Thus, it is important to choose a suitable sensitive material.
Metal-organic frameworks (MOFs), a burgeoning class of metal-coordinated cationic polymer nanomaterials consisting of metal cation aggregates which are linked by organic ligand molecules or metal cations, have large specific surface areas, tunable and porous structures, and high structural stability. Furthermore, MOFs also show excellent adsorption capacity and low environmental toxicity, making them promising candidates for environmental applications such as adsorption and catalysis [31,32]. However, MOFs are generally electrical insulators with poor conductivity. This greatly hinders their usage for chemiresistive gas sensing materials due to the foreseeable intrinsic low response values. In this case, pristine MOFs have been combined with other conductive materials to form heterostructures, either doped with heteroatoms, or simply calcined at high temperature to generate metal oxide-carbon composites (i.e., MOF derivatives) to improve their sensing performance [33,34]. For example, a uniform three-dimensional MXene/MOF composite, Ti 3 C 2 T X /ZIF-67/CoV 2 O 6 , has been obtained by a co-precipitation reaction, which integrated the conductivity of MXene and the redox activity of the MOF [35]. Generally, MOFs are suitable precursor materials which can be used as an ideal template to synthesize porous metal nano-oxides with an ideal structure. In this regard, their derivatives, such as metal oxides, carbon materials, and their composites, are often used as gas sensing materials due to their high chemical stability, rich structural diversity, large specific surface areas with highly porous structures, considerable temperature resistance, low environmental and physiological toxicity, and many other advantages [19,[36][37][38][39]. Furthermore, the doping of heteroatoms into MOFs is also a method for improving sensing performance. Doping heteroatoms can reduce the particle size of the sensing material by inducing the formation of oxygen vacancies, increasing the contact area and enhancing the sensing performance [40][41][42][43]. In addition, the ion or compound activation centers formed by doping heteroatoms can be used as catalysts in chemical reactions between detected gases and sensing materials, accelerating reactions and improving the selectivity and response speeds of sensing materials at a low temperature. Thus, doping heteroatoms into metal oxidebased sensing materials is beneficial for improving their gas-sensing properties [33][34][35][36][37][38][39][40][41][42][43][44][45][46][47]. For example, Zhang et al. synthesized Cu-In 2 O 3 hollow nanofibers by doping Cu atoms into In 2 O 3 and used them as H 2 S-sensing materials. Their study showed that the doping of Cu atoms and the formation of their hollow structure increased the number of active sites, and the small amount of CuO formed by Cu oxidation could combine with In 2 O 3 to synthesize the p-n heterojunction, greatly improving selectivity and responsiveness to H 2 S [48]. In addition, the doped heteroatoms could also accelerate the adsorption and desorption of oxygen to electrons, thus improving response performance [49]. In this paper, we summarize the methods of enhancing the sensing performance of chemiresistive gas sensors for harmful gases, such as by changing the morphology and structure of MOFs, doping heteroatoms, and other design methods.

Sensing Principles of Chemiresistive Gas Sensors
The chemiresistive gas sensor, an important part of the gas analysis system, is an instrument which can determine the concentration and composition of detected gas and convert the obtained chemical information into electrical information [50,51]. Chemiresistive gas sensors are often used to detect inorganic small molecular gases and VOCs. Chemiresistive gas sensors react differently when exposed to different inorganic small molecule gases. If exposed to a gas with strong oxidizing or strong reducing properties, the substances in the sensor and the analyte exchange electrons and holes or form a heterojunction to affect the resistance. If exposed to acid and alkaline gas, the substance in the sensor affects the resistance through the occurring chemical adsorption and chemical reaction with the analyte [52]. VOC detection mostly uses polymer as the gas-sensing material of the chemiresistive gas sensor. If exposed to a non-conductive polymer, the sensitive film begins to expand, the conductive path becomes longer, electron transfer becomes more difficult, and the resistance value increases after the polymer absorbs VOCs. However, the resistance value is changed via electron-hole exchange, forming a heterojunction if exposed to conductive polymers [53][54][55][56][57][58]. In previous studies, the performance of chemiresistive sensors has usually been evaluated by the following parameters: (1) Response (R): R = (R air − R gas )/R gas , where R gas and R air are the resistance with and without the presence of gaseous analyte; (2) Sensitivity (S): S = δR/δC t , where Ct is the concentration of the measured gas; (3) Repeatability: Repeatability is measured by comparing the response of the same device to the same concentration of measured gas for multiple cycles; (4) Stability: Stability refers to testing the gas-sensing performance of the same device to the measured gas after being placed at different times, comparing the change in gas sensitivity with time; (5) Selectivity: Selectivity of the gas sensor is evaluated by comparing the response gap of the same device to different gases with the same concentration.

Chemiresistive Gas Sensors Using MOF Derivatives
MOF derivatives-based chemiresistive sensors have attracted wide attention. They could simply be synthesized by thermochemical methods such as pyrolysis, using MOF as the template compromising the different functional units including metal ions and carbon species [59].

NO 2 Sensors
NO 2 , a kind of hazardous vehicle emission product and combustion product of fossil fuels with an acidic nature, can cause many environmental problems, such as acid rain, photochemical smog, haze, and water eutrophication [60][61][62]. Thus, it is very important to develop gas sensors with high sensing performance for NO 2 detection. Studies to date have shown that nanostructured metal oxides have higher sensing performance for NO 2 [63,64]. Ren et al. used a Zn-based zeolitic imidazolate framework (ZIF-8) as a template to synthesize porous ZnO nanocubes for the detection of NO 2 (Figure 1a) [65]. The experiment indicated that as the temperature increased, the organic bonds of the compounds were gradually pyrolyzed, the organic ligands were removed, and the metal nodes were oxidized to metal oxides leaving voids, as demonstrated by scanning electron microscopy (SEM) images of ZIF-8 and ZIF-8 derivatives at different pyrolysis temperatures (Figure 1b,c). By comparing the response of ZIF-8 derivatives synthesized at different pyrolysis temperatures to 1 ppm NO 2 , it was noted that ZIF-8-500 synthesized at the pyrolysis temperature of 500 • C had the highest response ( Figure 1e). Previous studies have confirmed that the charge transfer between absorbed gas and the gas sensor may affect the sensing performance of metal oxide gas sensors. When the gas sensor is exposed to NO 2 , NO 2 adsorbs on the material and takes electrons from the CB of ZnO, causing a resistance increase and producing an electron depletion layer (Figure 1f). Porous ZnO nanocubes inherit the high specific surface area of ZIF-8, and its unique porous hollow polyhedral structure creates numerous gas channels, making it easier for NO 2 to adsorb on its surface and take away electrons. Compared with normal ZnO, porous ZnO nanocubes present high sensitivity and a lower NO 2 detection limit. In addition, the prepared gas sensor exhibits good selectivity for NO 2 compared with other gases (CO, C 2 H 5 OH, H 2 , H 2 S, NO, NH 3 ). This is attributed to the excellent microstructure and surface states of the nanomaterials. that the charge transfer between absorbed gas and the gas sensor may affect the sensing performance of metal oxide gas sensors. When the gas sensor is exposed to NO2, NO2 adsorbs on the material and takes electrons from the CB of ZnO, causing a resistance increase and producing an electron depletion layer ( Figure 1f). Porous ZnO nanocubes inherit the high specific surface area of ZIF-8, and its unique porous hollow polyhedral structure creates numerous gas channels, making it easier for NO2 to adsorb on its surface and take away electrons. Compared with normal ZnO, porous ZnO nanocubes present high sensitivity and a lower NO2 detection limit. In addition, the prepared gas sensor exhibits good selectivity for NO2 compared with other gases (CO, C2H5OH, H2, H2S, NO, NH3). This is attributed to the excellent microstructure and surface states of the nanomaterials. Materials Institute Lavoisier (MIL)-based materials synthesized from terephthalic acid and metal-centered octahedron (MO4(OH)2, M = In, Ga, Fe), which are organic ligands, have a three-dimensional network structure with ultra-high porosity [66]. Du et al. took MIL(M = In) as a precursor doped with a small amount of Fe ions to synthesize Fe-In2O3 nanorods through thermochemical methods and a pyrolysis process, investigating its NO2 sensing performance (Figure 2a) [44]. By observing the SEM images of In/Fe-MIL-68s with different levels of Fe(III) content, all the samples showed a hexagonal rod-shaped architecture, the Fe-doped In2O3 nanorods had pores on the surface, and the structural size was slightly reduced, which was due to the shrinkage and decomposition of the MOF structure caused by the pyrolysis process. In comparing the high-resolution transmission Materials Institute Lavoisier (MIL)-based materials synthesized from terephthalic acid and metal-centered octahedron (MO 4 (OH) 2 , M = In, Ga, Fe), which are organic ligands, have a three-dimensional network structure with ultra-high porosity [66]. Du et al. took MIL (M = In) as a precursor doped with a small amount of Fe ions to synthesize Fe-In 2 O 3 nanorods through thermochemical methods and a pyrolysis process, investigating its NO 2 sensing performance ( Figure 2a) [44]. By observing the SEM images of In/Fe-MIL-68s with different levels of Fe(III) content, all the samples showed a hexagonal rod-shaped architecture, the Fe-doped In 2 O 3 nanorods had pores on the surface, and the structural size was slightly reduced, which was due to the shrinkage and decomposition of the MOF structure caused by the pyrolysis process. In comparing the high-resolution transmission electron microscopy (HRTEM) images of Fe(0)-In 2 O 3 , it was found that doping Fe(III) forms lattice defects because Fe(III) replaces the In 3+ ions in the crystal structure (Figure 2d,e). According to Figure 2f,g, Fe(5)-In 2 O 3 nanorods showed excellent responsiveness and selectivity to NO 2 compared with similar products because the nanorod exhibits superior NO 2 gas-sensing performance at low temperatures due to the high diffusivity, multiple active sites, and wide depletion layer brought by its unique structure. When the sensor is exposed to air, O 2 adsorbs on the surface of the nanorods and extracts the electrons in the sensing material, forming O − or O 2− , which leads to the formation of a potential barrier by bending the energy band and increasing resistance, in turn reflecting the sensing performance of the material for NO 2 . When the sensor is exposed to NO 2 gas, it contacts and combines with the electrons of the sensing material to form NO 2 − , resulting in further band bending, higher potential barrier formation, and the further increase in resistance (Figure 2h). In summary, in order to better adsorb and sense the NO 2 molecules, the MOF precursors with high specific surface areas should be selected, and the resulting metal oxide composites are usually doped to improve conductivity, and thereby to improve the electron exchange and sensitivity toward NO 2 . electron microscopy (HRTEM) images of Fe(0)-In2O3, it was found that doping Fe(III) forms lattice defects because Fe(III) replaces the In 3+ ions in the crystal structure ( Figure  2d,e). According to Figure 2f,g, Fe(5)-In2O3 nanorods showed excellent responsiveness and selectivity to NO2 compared with similar products because the nanorod exhibits superior NO2 gas-sensing performance at low temperatures due to the high diffusivity, multiple active sites, and wide depletion layer brought by its unique structure. When the sensor is exposed to air, O2 adsorbs on the surface of the nanorods and extracts the electrons in the sensing material, forming O − or O 2− , which leads to the formation of a potential barrier by bending the energy band and increasing resistance, in turn reflecting the sensing performance of the material for NO2. When the sensor is exposed to NO2 gas, it contacts and combines with the electrons of the sensing material to form NO2 − , resulting in further band bending, higher potential barrier formation, and the further increase in resistance ( Figure 2h). In summary, in order to better adsorb and sense the NO2 molecules, the MOF precursors with high specific surface areas should be selected, and the resulting metal oxide composites are usually doped to improve conductivity, and thereby to improve the electron exchange and sensitivity toward NO2.  With the rapid development of electronic products, wearability and high selectivity have attracted significant attention. In a study by Bag et al., a NO 2 sensor based on reduced graphene oxide (rGO)-ZnFe 2 O 4 was developed by uniformly anchor smearing MOFderived mesoporous ZnFe 2 O 4 microparticles on the rGO sensor layer [67,68]. Because of the synergistic reaction between the mesoporous ZnFe 2 O 4 particles and the rGO sensing layer, the gas sensor showed improved mechanical stretchability and signal stability compared with rGO-only devices, exhibited superior response and sensitivity to NO 2 , and had good repeatability and selectivity even under high humidity conditions, which is expected to be applied in future wearable electronics.

Acetone Sensors
Acetone, a harmful VOC, is widely used in the chemical industry and chemical experiments. Trace amounts of acetone gas can cause great harm to the environment and to human health, including headache, coma, and even death. Moreover, acetone is one of the important indicators used for diabetes detection [69][70][71]. Thus, it is very important to develop efficient and sensitive acetone gas sensors. Zhu et al. used Fe-MIL-88B-NH 2 as a precursor and MEMS as a substrate to synthesize Fe 2 O 3 /C mesoporous nanorods (NR) via simple hydrothermal and pyrolysis reactions, and investigated its acetone-sensing properties [72,73]. The experiment indicated that when the calcination temperature was lower than 500 • C, as the temperature increased, α-Fe 2 O 3 was gradually formed in the compounds, the crystallinity was increased, and the organic molecules were gradually carbonized and decomposed, as shown in the SEM images of the Fe-MOF precursors and their derivatives at different calcination temperatures (Figure 3a,b). Furthermore, the specific surface area of the mesoporous NRs increased significantly after calcination at 300 • C, which was beneficial to their acetone gas-sensing properties. In addition, a hollow structure was formed in the compound, which might have been caused by the lattice rearrangement and the complete decomposition of the organic ligands when the calcination temperature reached 400 • C, as displayed in the TEM images (Figure 3c,d). When the synthesized material was exposed to acetone, the electrons on the Fe 2 O 3 surface were captured by the oxygen adsorbed on it, resulting in an increase in the depletion layer and the potential barrier. The oxygen species adsorbed on its surface reacted with acetone, changing the resistance (Figure 3g). The study showed that the synthesized carbon nanoparticle-modified mesoporous α-Fe 2 O 3 NRs exhibited excellent thermal stability, accurate selectivity, and a fast response to acetone (Figure 3e,f), which may have been due to its large specific surface area and excellent electrical conductivity. With the rapid development of electronic products, wearability and high selectivity have attracted significant attention. In a study by Bag et al., a NO2 sensor based on reduced graphene oxide (rGO)-ZnFe2O4 was developed by uniformly anchor smearing MOF-derived mesoporous ZnFe2O4 microparticles on the rGO sensor layer [67,68]. Because of the synergistic reaction between the mesoporous ZnFe2O4 particles and the rGO sensing layer, the gas sensor showed improved mechanical stretchability and signal stability compared with rGO-only devices, exhibited superior response and sensitivity to NO2, and had good repeatability and selectivity even under high humidity conditions, which is expected to be applied in future wearable electronics.

Acetone Sensors
Acetone, a harmful VOC, is widely used in the chemical industry and chemical experiments. Trace amounts of acetone gas can cause great harm to the environment and to human health, including headache, coma, and even death. Moreover, acetone is one of the important indicators used for diabetes detection [69][70][71]. Thus, it is very important to develop efficient and sensitive acetone gas sensors. Zhu et al. used Fe-MIL-88B-NH2 as a precursor and MEMS as a substrate to synthesize Fe2O3/C mesoporous nanorods (NR) via simple hydrothermal and pyrolysis reactions, and investigated its acetone-sensing properties [72,73]. The experiment indicated that when the calcination temperature was lower than 500 °C, as the temperature increased, α-Fe2O3 was gradually formed in the compounds, the crystallinity was increased, and the organic molecules were gradually carbonized and decomposed, as shown in the SEM images of the Fe-MOF precursors and their derivatives at different calcination temperatures (Figure 3a,b). Furthermore, the specific surface area of the mesoporous NRs increased significantly after calcination at 300 °C, which was beneficial to their acetone gas-sensing properties. In addition, a hollow structure was formed in the compound, which might have been caused by the lattice rearrangement and the complete decomposition of the organic ligands when the calcination temperature reached 400 °C, as displayed in the TEM images (Figure 3c,d). When the synthesized material was exposed to acetone, the electrons on the Fe2O3 surface were captured by the oxygen adsorbed on it, resulting in an increase in the depletion layer and the potential barrier. The oxygen species adsorbed on its surface reacted with acetone, changing the resistance (Figure 3g). The study showed that the synthesized carbon nanoparticlemodified mesoporous α-Fe2O3 NRs exhibited excellent thermal stability, accurate selectivity, and a fast response to acetone (Figure 3e,f), which may have been due to its large specific surface area and excellent electrical conductivity. Zhang et al. synthesized a ZnO/Co 3 O 4 nano-heterostructure using ZIF-8/ZIF-67 as a precursor via a facile co-precipitation method [74]. The experiment results demonstrated that ZnO nanopolyhedrons are composed of many small pores. ZnO/Co 3 O 4 nanopolyhedrons exhibit a similar hollow polyhedron structure with ZnO nanopolyhedrons revealed by the SEM and TEM images (Figure 4a,b). The response diagrams of the ZnO and ZnO/Co 3 O 4 thin film sensors to acetone were further observed (Figure 4c,d) in the experiment, which showed that the ZnO/Co 3 O 4 thin film sensor exhibited good selectivity, reproducibility, repeatability, and stability to acetone. When ZnO is exposed to air, oxygen molecules adsorb on the surface of ZnO nanopolyhedrons and obtain electrons to form O 2− and O − ions, forming a further depletion layer. When ZnO was exposed to acetone, the acetone reacted with the adsorbed oxygen ions, thereby reducing the height and width of the depletion layer and further reducing the resistance of the sensor (Figure 4e). The sensing mechanism of the ZnO/Co 3 O 4 acetone sensor is similar to that of ZnO, while the sensing performance is greatly improved with regard to response, repeatability, and selectivity. This may be facilitated by the superior catalytic performance of Co 3 O 4 , the p-n heterojunction formed between ZnO and Co 3 O 4 , and the unique hollow structure. In general, sophisticated metal oxide-based heterostructures with enhanced porosity and lattice rearrangement, or with a favorable p-n heterojunction, could be obtained from designed MOFs as the template, which could consequently achieve a rapid response to acetone as a result, being attributable to the improved carrier dynamics [75][76][77][78].
Zhang et al. synthesized a ZnO/Co3O4 nano-heterostructure using ZIF-8/ZIF-67 as a precursor via a facile co-precipitation method [74]. The experiment results demonstrated that ZnO nanopolyhedrons are composed of many small pores. ZnO/Co3O4 nanopolyhedrons exhibit a similar hollow polyhedron structure with ZnO nanopolyhedrons revealed by the SEM and TEM images (Figure 4a,b). The response diagrams of the ZnO and ZnO/Co3O4 thin film sensors to acetone were further observed (Figure 4c,d) in the experiment, which showed that the ZnO/Co3O4 thin film sensor exhibited good selectivity, reproducibility, repeatability, and stability to acetone. When ZnO is exposed to air, oxygen molecules adsorb on the surface of ZnO nanopolyhedrons and obtain electrons to form O 2− and O − ions, forming a further depletion layer. When ZnO was exposed to acetone, the acetone reacted with the adsorbed oxygen ions, thereby reducing the height and width of the depletion layer and further reducing the resistance of the sensor (Figure 4e). The sensing mechanism of the ZnO/Co3O4 acetone sensor is similar to that of ZnO, while the sensing performance is greatly improved with regard to response, repeatability, and selectivity. This may be facilitated by the superior catalytic performance of Co3O4, the p-n heterojunction formed between ZnO and Co3O4, and the unique hollow structure. In general, sophisticated metal oxide-based heterostructures with enhanced porosity and lattice rearrangement, or with a favorable p-n heterojunction, could be obtained from designed MOFs as the template, which could consequently achieve a rapid response to acetone as a result, being attributable to the improved carrier dynamics [75][76][77][78].

Ethanol Sensors
Zhang et al. attempted to synthesize solid, hollow, and hierarchical hollow nanocages with quantum dots (HHQD) of ZnO for ethanol gas sensing [79]. The HHQD-

Ethanol Sensors
Zhang et al. attempted to synthesize solid, hollow, and hierarchical hollow nanocages with quantum dots (HHQD) of ZnO for ethanol gas sensing [79]. The HHQD-ZnO nanocages were synthesized from the ZIF-8 product with a size of 170 mm (170-ZIF-8) as a precursor, and the SEM images of the 170-ZIF-8 nanocrystals (Figure 5a,b) indicated that the synthesized 170-ZIF-8 precursors had a uniform cage-like morphology with good connectivity between particles. The further study of the TEM images revealed (Figure 5c,d) that the formed HHQD-ZnO nanocages inherited the cage-like morphology of the precursor, presenting a large-area hollow structure and a large specific surface area. According to the response diagram of the HHQD-ZnO nanocages to ethanol gas (Figure 5e), the response value of the HHQD-ZnO nanocage sensor to ethanol was much higher than that of the solid ZnO nanocage sensor, and the sensor showed high selectivity to ethanol gas (Figure 5f), which was due to the unique hollow interpenetrating nanostructure and large specific surface area. When the HHQD-ZnO nanocage sensor was exposed to ethanol, its hollow nanocage structure could adsorb oxygen and carry away free electrons from the CB, thereby increasing the potential barrier and resulting in resistance change.
8) as a precursor, and the SEM images of the 170-ZIF-8 nanocrystals (Figure 5a,b) indicated that the synthesized 170-ZIF-8 precursors had a uniform cage-like morphology with good connectivity between particles. The further study of the TEM images revealed (Figure  5c,d) that the formed HHQD-ZnO nanocages inherited the cage-like morphology of the precursor, presenting a large-area hollow structure and a large specific surface area. According to the response diagram of the HHQD-ZnO nanocages to ethanol gas (Figure 5e), the response value of the HHQD-ZnO nanocage sensor to ethanol was much higher than that of the solid ZnO nanocage sensor, and the sensor showed high selectivity to ethanol gas (Figure 5f), which was due to the unique hollow interpenetrating nanostructure and large specific surface area. When the HHQD-ZnO nanocage sensor was exposed to ethanol, its hollow nanocage structure could adsorb oxygen and carry away free electrons from the CB, thereby increasing the potential barrier and resulting in resistance change.

H2S Sensors
H2S is a flammable and toxic gas which can be produced in the production processes of food processing plants, paper mills, oil refineries, and other factories, and can also be produced by gas combustion and after the decomposition of human and animal excreta [80][81][82][83][84]. H2S can affect the human nervous system and visual system, causing Alzheimer's disease, loss of consciousness, and other problems [85]. Thus, it is urgent that H2S detection and monitoring sensing systems are improved in order to reasonably control H2S emissions [86,87]. Li et al. first synthesized CPP-3 (In) microrods and then used Cu 2+ -impregnated CPP-3 (In) microrods impregnated with Cu 2+ as MOF precursors to synthesize bamboo-like CuO/In2O3 heterostructures via heating, cooling, centrifugation, drying, and calcination ( Figure 6a) [88]. SEM (Figure 6b) and TEM images (Figure 6c) of CuO/In2O3 showed that the synthesized CuO/In2O3 inherited the rod-like shape of the CPP-3(In) precursor, but the average size was slightly reduced, showing a bamboo-like hollow structure. In addition, the images also showed the existence of p-n heterojunctions formed between CuO and In2O3 nanoparticles. By comparing the response values of CuO/In2O3 to different gases (Figure 6d,e), CuO/In2O3 showed excellent selectivity and responsiveness to H2S at low temperatures. Intrinsically, CuO is a p-type compound and In2O3 is an n-

H 2 S Sensors
H 2 S is a flammable and toxic gas which can be produced in the production processes of food processing plants, paper mills, oil refineries, and other factories, and can also be produced by gas combustion and after the decomposition of human and animal excreta [80][81][82][83][84]. H 2 S can affect the human nervous system and visual system, causing Alzheimer's disease, loss of consciousness, and other problems [85]. Thus, it is urgent that H 2 S detection and monitoring sensing systems are improved in order to reasonably control H 2 S emissions [86,87]. Li et al. first synthesized CPP-3 (In) microrods and then used Cu 2+impregnated CPP-3 (In) microrods impregnated with Cu 2+ as MOF precursors to synthesize bamboo-like CuO/In 2 O 3 heterostructures via heating, cooling, centrifugation, drying, and calcination ( Figure 6a) [88]. SEM (Figure 6b) and TEM images (Figure 6c) of CuO/In 2 O 3 showed that the synthesized CuO/In 2 O 3 inherited the rod-like shape of the CPP-3(In) precursor, but the average size was slightly reduced, showing a bamboo-like hollow structure. In addition, the images also showed the existence of p-n heterojunctions formed between CuO and In 2 O 3 nanoparticles. By comparing the response values of CuO/In 2 O 3 to different gases (Figure 6d,e), CuO/In 2 O 3 showed excellent selectivity and responsiveness to H 2 S at low temperatures. Intrinsically, CuO is a p-type compound and In 2 O 3 is an n-type compound. When CuO came into contact with In 2 O 3 , holes in the CuO and electrons in the In 2 O 3 flowed in opposite directions, forming an internal electric field, an energy band bending in the depletion layer, and subsequently forming a p-n heterojunction, enhancing gas-sensing performance (Figure 6f,g). Finally, the unique mesoporous bamboo-like hollow structure and the facilitated electron transfer resulted from the p-n heterojunction, and together enabled the CuO/In 2 O 3 sensor superior selectivity, reproducibility, and sensitivity to H 2 S. Furthermore, Karuppasamy et al. synthesized Ni 4 Mo/MoO 2 @C composite nanospheres via a co-precipitation and high-temperature calcination process using MOF as a precursor [89]. The gas response of the Ni 4 Mo/MoO 2 @C composite nanospheres to H 2 S was 3.5 times and 2.6 times higher than those of Ni-MOF and Mo-MOF, respectively, which was attributed to the synergistic effect of the Ni 4 Mo/MoO 2 @C composite nanospheres and the high surface area derived from the unique morphology. Overall, the p-n heterojunction with well-defined energy level bending can be introduced into the depletion layer with the reverse flow of holes and electrons, which could afford a reliable detection of H 2 S at a lower power consumption [90][91][92]. type compound. When CuO came into contact with In2O3, holes in the CuO and electrons in the In2O3 flowed in opposite directions, forming an internal electric field, an energy band bending in the depletion layer, and subsequently forming a p-n heterojunction, enhancing gas-sensing performance (Figure 6f,g). Finally, the unique mesoporous bamboolike hollow structure and the facilitated electron transfer resulted from the p-n heterojunction, and together enabled the CuO/In2O3 sensor superior selectivity, reproducibility, and sensitivity to H2S. Furthermore, Karuppasamy et al. synthesized Ni4Mo/MoO2@C composite nanospheres via a co-precipitation and high-temperature calcination process using MOF as a precursor [89]. The gas response of the Ni4Mo/MoO2@C composite nanospheres to H2S was 3.5 times and 2.6 times higher than those of Ni-MOF and Mo-MOF, respectively, which was attributed to the synergistic effect of the Ni4Mo/MoO2@C composite nanospheres and the high surface area derived from the unique morphology. Overall, the p-n heterojunction with well-defined energy level bending can be introduced into the depletion layer with the reverse flow of holes and electrons, which could afford a reliable detection of H2S at a lower power consumption [90][91][92].

Other Gas Sensors
Zhang et al. synthesized NiFe2O4 nano-octahedrons via the direct pyrolysis of NiFebimetallic MOFs to explore its gas sensing performance to toluene [93]. The study showed that the sensor exhibited a low recovery time, strong stability, high repeatability, and low detection, which was due to the catalytic properties and high specific surface area resulting from its porous structure. Qin et al. synthesized Co3O4 dodecahedrons by calcining a Co-MOF template. The sensing material exhibited good selectivity and high responsiveness to CO because of the large number of Co 3+ active sites and surface adsorption of oxygen [94].

Other Gas Sensors
Zhang et al. synthesized NiFe 2 O 4 nano-octahedrons via the direct pyrolysis of NiFebimetallic MOFs to explore its gas sensing performance to toluene [93]. The study showed that the sensor exhibited a low recovery time, strong stability, high repeatability, and low detection, which was due to the catalytic properties and high specific surface area resulting from its porous structure. Qin et al. synthesized Co 3 O 4 dodecahedrons by calcining a Co-MOF template. The sensing material exhibited good selectivity and high responsiveness to CO because of the large number of Co 3+ active sites and surface adsorption of oxygen [94].

Conclusions and Future Perspectives
The chemiresistors, a type of chemical gas sensor, can detect the concentration changes of target gases through changes in resistance signals. Studies have shown that the performance of chemiresistors could be promoted by selecting suitable gas-sensing materials to achieve high responsiveness and selectivity to the specific target gas. MOF derivatives synthesized with MOFs as precursors hold great potential in gas sensing due to their unique structures, high selectivity, sensitivity, and versatility. In this review, we summarized the applications of MOF derivatives for sensing NO 2 , acetone, ethanol, H 2 S, and several other toxic gases, with their detailed sensing performance and mechanisms being described and discussed.
However, although the novel MOF derivatives-based chemiresistors suggest many new opportunities in in situ detection and monitoring of the harmful gases in industries and in daily life, there are still several challenging points which need to be dealt with: (1). The MOF derivatives should successfully maintain or inherit the original high porosity and redox activity of pristine MOFs during the high-temperature pyrolysis process so that they can achieve excellent sensitivity and response as a chemiresistive gas sensor; (2). The sophisticated morphologies and precisely tailored physicochemical properties of the MOF derivatives need to be constructed and established by thermochemical or other methods, avoiding from the unfavorable Ostwald ripening process, in order to increase the active surface areas affording adsorption and catalysis reaction with gaseous molecules, and also the surface electron affinity to enhance their resistance changing signals; (3). Efficient charge transfer needs to be realized by the construction of a p-n junction and other heterojunction interfaces so that rapid response and recovery times are available for the chemiresistive gas sensors; (4). The reproducibility and cost control for the preparation of MOF derivatives and the as-resulted chemiresistive sensing devices are still far from satisfactory; (5). The realization of self-powered, minimized and potable gas sensing devices based on MOF derivatives is another indispensable future research direction, especially with the rapid development of 5G wireless networks currently taking place; (6). Finally, the rapid and efficient data transmission and establishment of the gas sensorsbased IoT system, which are of great significance to safer and cleaner production by avoiding the leakage of toxic, harmful, flammable and explosive gases like methane leakage during the exploitation of oil and natural gas, also require the further utilization and optimization of the MOF derivatives-based chemiresistive gas sensors.